Linux Serial Console

These examples are valid if you want to use /dev/ttyS1 (COM2)
as the serial console. Replace as needed.

1. Tell LILO to use the serial port.
   In lilo.conf (global section): 

   serial  = 1,9600n8 (ttyS1, 9600 bd, no parity, 8 bits)

2. Adjust to kernel flags for the new kernel,
   again in lilo.conf (kernel section)

   append = "console=1,9600,n8" 

   (Note the extra comma needed if you want to supply parity/framing 
   information.)

3. Link /dev/console to the serial port.

   Your probably want to save your old /dev/console (the "master" virtual
   console). Check if it is a symbolic link first. If not, `mv' it to
   `/dev/tty0':

   ls -l /dev/console
   mv /dev/console /dev/tty0

   Now link the serial port you are going to use as the console to
   /dev/console, for example ttyS1:

   ln -s /dev/ttyS1 /dev/console

   On some systems you might want to edit your bootup scripts to make sure
   they don't reset this arrangement on boot. (On Debian, check
   /etc/rc.boot/console and /etc/default/console). You probably also want
   to put a getty on either /dev/console or /dev/ttyS1.

4. Init and /dev/console.
   Sysvinit will open /dev/console on boot. If this does not point
   to the serial console device, the startup messages will be printed
   to the wrong device. The kernel also passes the environment variable
   "CONSOLE" to the init program. sysvinit-2.64 reckognizes this, and
   opens that device instead. Boot scripts as mentioned in (3) can
   also check this variable to see what device the system console is.
   If CONSOLE is not set you can assume the console is /dev/tty0.

   Sysvinit remembers its stty settings in a file in /etc, called
   `/etc/ioctl.save'. REMOVE THIS FILE before using the serial
   console for the first time, because otherwise init will probably
   set the baudrate to 38400 (baudrate of the virtual console).

5. /dev/console and X
   Programs that want to do something with the virtual console usually
   open /dev/console. XF86 does this, and probably SVGALIB as well.
   IMO this is wrong; they should open /dev/tty0.
   I have binary patched /usr/bin/X11/XF86_SVGA to use "tty0"
   instead of "console".

6. Notes.

   If you compile the next little program, you will be able
   to really "halt" the system. It will enter a little
   monitor :)

   main() { reboot(0xfee1dead, 672274793, 0xCDEF0123); }

   This is just a call to the new "halt" function that later
   kernels have. This is included the "halt" command of
   the recent sysvinit versions.

   The monitor will also be entered at a kernel panic, or
   when you press "break". That last function does not
   work at the moment I think, but it would be useful for
   kernel debugging.  You don't have alt-scrollock on a serial
   console to find out the current EIP...

Miquel van Smoorenburg <miquels@cistron.nl>, 21-Jun-1996
Stephen C. Tweedie <sct@dcs.ed.ac.uk>, 23-Dec-1996